1. Field of the Invention
The present invention relates to an acoustic transducer of electromagnetic coupling type (electromagnetic induction type), i.e. a transducer such as a speaker or a headphone for converting an electrical signal into acoustic sounds and a transducer such as a microphone for converting acoustic sounds into an electrical signal.
2. Description of the Related Art
In the case of an external magnet type, for example, an acoustic transducer of an electromagnetic coupling type comprises a magnetic circuit having a magnetic gap formed between a plate and a center pole across a magnet composed of the plate, a yoke with the center pole attached thereto and in which a first coil is fixed to the plate or the center pole within the magnetic gap of the magnetic circuit and an insulated second coil is fixed to a diaphragm in an opposing relation to the first coil within the magnetic gap of the magnetic circuit.
In an acoustic transducer such as a speaker or a headphone, when a first coil is used as a drive coil (primary coil) and a signal current is supplied to this drive coil, a secondary current corresponding to the signal current is induced in a second coil serving as a secondary coil by an electromagnetic coupling. Then, owing to the Fleming's left-hand rule, a drive force corresponding to a signal current is generated in the second coil, and the diaphragm to which the second coil is fixed is vibrated to generate a sound pressure corresponding to the signal current.
FIGS. 14 and 15 show examples of electromagnetic coupling type speakers according to the related art, respectively. FIG. 14 shows the example of the electromagnetic coupling type speaker in which a drive coil is attached to a plate. FIG. 15 shows the example of the electromagnetic coupling type speaker in which a drive coil is attached to a center pole.
Specifically, in the electromagnetic coupling type speaker shown in FIG. 14, a center pole 11 is unitarily formed at the center portion of the upper surface of a flange-like yoke 10. A magnet 20 is attached to the upper surface of the circumferential portion of the yoke 10. An annular plate 30 is attached to the upper surface of the magnet 20. A magnetic circuit 6 is formed so as to have a magnetic gap 5 formed between an outer peripheral surface of a tip end portion of the center pole 11 and an inner peripheral surface of the plate 30, and a drive coil 1 is attached to the inner peripheral end surface of the plate 30.
The yoke 10 has a hole 12 defined at its bottom portion and also has a terminal assembly 4 with an input terminal 3 attached to its lower surface. A lead wire 2 of the drive coil 1 is inserted into the hole 12 and connected to the input terminal 3 by soldering. The lead wire 2 is each attached to the start of the winding and the end of the winding of the drive coil 1, and each connected to a separate input terminal.
A secondary coil 7 is inserted into the magnetic gap 5. The secondary coil 7 is either an insulated cylinder of one turn made of a nonmagnetic conductive material such as aluminum or an insulated winding having a plurality of turns.
A lower portion of a frame 40 is attached to the upper surface of the plate 30. An outer peripheral portion of an upper end of a diaphragm 50 such as a cone is attached through an edge 51 and a gasket 45 to an upper inner peripheral end portion of the frame 40. An outer peripheral portion of a damper 47 is attached to the frame 40, and a lower end portion of the diaphragm 50 and an inner peripheral portion of the damper 47 are attached to the secondary coil 7. A center cap 49 is attached to a lower end portion of the diaphragm 50 or an upper end portion of the secondary coil 7.
In the electromagnetic coupling type speaker shown in FIG. 15, a recess is formed around the outer peripheral surface of the upper end portion of the center pole 11, and the drive coil 1 is attached to the center pole 11 by means of this recess. A rest of elements and parts in FIG. 15 is similar to that of the electromagnetic coupling type speaker shown in FIG. 14.
In the electromagnetic coupling type speaker shown in FIG. 14 or 15, when a signal current is supplied to the drive coil 1, a secondary current corresponding to the signal current is induced in the secondary coil 7 due to an electromagnetic coupling. Then, owing to the Fleming's left-hand rule, a drive force corresponding to the signal current is generated in the secondary coil 7, and the diaphragm 50 with the secondary coil 7 attached thereto is vibrated in the upper and lower direction, thereby resulting in a sound pressure corresponding to the signal current being generated.
However, in the related-art electromagnetic coupling type speaker shown in FIG. 14 or 15, since the drive coil 1 is disposed within the magnetic gap 5 of the magnetic circuit 6, the width (length of the direction perpendicular to the axis of the speaker) of the magnetic gap 5 cannot be reduced by the thickness of the drive coil 1 so that a magnetic force of the magnetic gap 5 is reduced, thereby resulting in a sensitivity of the speaker being lowered. If a large magnet is used as the magnet 20 in order to increase the magnetic force of the magnetic gap 5 and to increase the sensitivity of the speaker, then the speaker becomes large in size and cannot be produced inexpensively.
In addition, if the number of turns of the drive coil increases in order to increase the inductance of the drive coil 1, then the width of the magnetic gap 5 increases so that the sensitivity of the speaker is lowered. Hence, the inductance of the drive coil 1 cannot increase. As a result, an electromagnetic coupling force between the drive coil 1 and the secondary coil 7 is too lowered in a low band range of less than 2 kHz to reproduce low-frequency signals of large amplitude. Hence, the electromagnetic coupling speaker according to the related art can be used only to reproduce high-frequency signals.
Furthermore, while the outer or inner circumferential surface of the drive coil 1 contacts with the plate 30 or the center pole 11, its contact area is small so that heat cannot be radiated from the drive coil 1 instantly. As a consequence, not only a thick wire material cannot be used as the drive coil 1 but also a large current cannot be quickly flowed to the drive coil 1 with the result that an allowable input signal level cannot be increased.
While the case in which the electromagnetic coupling type transducer is applied to the speaker has been described so far, this is also true in other transducer such as the headphone. The transducer such as the microphone has a similar arrangement except only that the input and output are reversed.